We won't know until T-Mobile's device is launched. Google has said that carrier's will be able to customize their handset. To what degree? We don't know.

Having the Android SDK and seeing Android run on a live device that is on the market are two incredibly different things. To be honest, you'll just have to wait.

You can bet that as soon as Android is launched there will be a gang of people working on doing whatever type of "unlocking" they possibly can if it isn't completely open. Similar to the iPhone pwners.